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WASHINGTON — The Interior Department said Thursday the survival of sea otters in southwest Alaska is threatened and proposed adding them to the government’s endangered species list.

If the proposal were adopted, it would lead to a recovery plan requiring conservation efforts for the northern sea otter. It inhabits waters in the western Gulf of Alaska stretching toward the Bering Sea, including the Alaska Peninsula, Aleutian Islands and Kodiak Island.

“No one is certain yet what is causing this, but listing this population as ’threatened’ under the Endangered Species Act will be an important step in discovering the reasons and reversing the decline,” Interior Secretary Gale Norton said.

Follows lawsuitIn December, two animal welfare groups sued the department’s Fish and Wildlife Service in U.S. District Court in San Francisco to have sea otters added to the endangered species list. The Arizona-based Center for Biological Diversity petitioned for such a listing in 2000.

The Fish and Wildlife Service shares responsibility for protecting endangered species with the Commerce Department’s National Marine Fisheries Service. Threatened species are considered likely to become endangered; endangered species are thought to be in jeopardy of extinction.

Commercial hunting from the mid-1700s to the early 1900s drove sea otters to near-extinction in southwest Alaska. They began recovering after commercial harvests were banned under a 1911 international treaty.

By the mid-1980s, about 55,000 to 74,000 sea otters inhabited southwest Alaska — almost half the world’s total. Since then, aerial surveys suggest the population has fallen again by at least 55 percent, and possibly as much as 67 percent, and that the trend is continuing.

Possible causesScientists say some evidence points to killer whales preying on sea otters as a cause. They also have found that sea otters ingested hydrocarbons and suffered liver and tissue damage from crude oil still buried under shoreline where the tanker Exxon Valdez ran aground in 1989. So far, they have not found evidence of harm from other potential factors like declining reproductivity, starvation, disease, contaminants or commercial fishing.

In the first two years of the Bush administration, the Fish and Wildlife Service added 21 species to the endangered species list, Fish and Wildlife officials said.